The German Patent Specification DE 199 40 911 C1 discloses a tree grille system, in which several grille sections are interconnected by means of couplings to form a continuous, self-supporting tree grille. The couplings consist of two fixed coupling parts configured on the grille sections and a coupling part, which can be inserted between the two fixed coupling parts and then rotated into a locking position. The fixed coupling parts are formed by segment pieces, which comprise a ramp-type clamping surface on the underside. The displaceable coupling part consists of a discoidal head, a bolt-shaped center part and a two-armed crossbar, the two end regions of which are directed upwardly and can be brought into active connection with the ramp-type clamping surface of both the fixed coupling parts. A part of these ramp-type clamping surfaces is provided with a flat undulating or dented fluting, to which a flat tooth, which can be engaged in the depressions of the fluting, is assigned on each of the end regions of the crossbars. The aim of this configuration of the coupling is to lock the coupling part not only by a friction fit, but also by a form fit by engaging the teeth in the depressions when the displaceable coupling part is rotated into the locking position.
It has been seen that in the production of the grille sections consisting of cast iron, it is difficult with respect to casting technology, to manufacture and possibly rework a fluting provided on the underside of the ramp-type clamping surfaces, which extend at a flat angle to the plane of the grille sections. Furthermore, it is possible that the intended form-fit locking of the displaceable coupling part is achieved imperfectly if this coupling part is rotated into the locking position using an extremely small torque.